• Category: Science
  • Words: 3042
  • Grade: 94
Bacteria are an ancient and diverse form of life that have always been an integral part of the human body. The majority of bacteria are harmless, and some even helpful and necessary for the occurrence of specific bodily functions. However, some bacteria have proven to be harmful, and throughout the history of man our species has been plagued by bacterial infection. Pathogenic bacteria have many structural characteristics that allow them to survive and grow in a human host. At the same time, the human immune system must be capable of suppressing the growth of these foreign microorganisms in order to prevent the human body from becoming infected, and consequently has various methods for the destruction of harmful pathogens.
Bacteria in general have many different ways of affecting us, some positively, some negatively, others not at all. Pathogenic bacteria are the bacteria that affect us in a direct and negative way. Bacteria are commonly divided into two distinct groups: Gram positive and Gram negative bacteria. The gram reaction of a bacterial species has to do with their cell wall structure. Other differences can be observed between the two divisions however. The cell wall of a bacterium is important when dealing with the virulence of the specific microbe. There are two possible types of cell wall that a bacterium may posses depending on its' Gram reaction.
Gram negative bacteria have two cell membranes sandwiching a thin layer of peptidoglycan, and lipopolysaccharides in the outer membrane. These lipopolysaccharides contain lipid A, an endotoxin capable of triggering symptoms from a human host. The Gram negative cell will not expel any of the endotoxins until lysis occurs. When the bacterial cell wall deteriorates the endotoxins are released into the human host and it is only at this point that disease can begin to occur. The endotoxins from a gram negative cell all produce the same symptoms from the human host no matter what organism released the toxin. The symptoms will, however, vary in severity. Responses from the host as a direct result of endotoxins include chills, fevers, shock and in extreme cases death. Death is very rarely associated with Gram negative endotoxins, but is more common when looking at Gram positive exotoxins.
The Gram positive cell wall lacks an outer membrane, but has a much thicker layer of peptidoglycan on the outside of the cell membrane. Techoic acids are present on the outside of the cell wall. Exotoxins can be generally found in the Gram negative cell structure. These toxins are produced in some bacteria throughout the metabolic life span of the bacterial cell. The toxins are constantly being created and excreted into the extra cellular fluid of the cell. These exotoxins are made up of proteins whose gene responsible for the coding is usually carried on plasmids. The water-soluble proteins are expelled into the aqueous solutions of the human body where they are quickly dissolved and distributed throughout the body.
There are three types of exotoxins. Cytotoxins kill the host cells and affect their functions. Often the production of proteins in eucaryotic cells is interrupted by cytotoxins. Diphtheria is an example the result of a cytotoxin infection as it inhibits the protein production in the host cells. Neurotoxins interrupt the transmission of impulses in the nervous system. Tetanus or lockjaw is an example of the nerve-damaging toxin, as it contracts and seizes the muscles in the jaw before complicating bodily functions further. Enterotoxins affect the intestinal tract. Cholera results when polypeptides produced from Vibrio cholerae bind to the epithelial cells lining the tract and disturb normal muscular contractions resulting in diarrhea. Exotoxins produced by bacteria are some of the most lethal substances known to man. 1 mg of the toxin created by the causative agent of botulism is enough to kill a million small mammals the size of guinea pigs.
Fimbriae are commonly found on Gram negative pathogens. Fimbriae are composed of a protein called pilin, which coincidentally is also the protein that makes up pili. Often both structures are referred to as pili but differentiating the two is important as they both serve very different functions. Fimbriae are tiny hair-like appendages that can be found on either poles of the cell or covering the entire cell wall. Their numbers can range from a few to several hundred spread throughout the exterior of the cell wall. Fimbriae allow bacteria to attach to various surfaces, as well as to other bacteria, which is specifically known as colonization. The colonization of bacteria on some surfaces would not be possible if fimbriae were not present on the bacterial cell. The colonization of bacteria is in some cases directly related to the pathogenicity of the microbe, as the huge numbers of bacterial cells present in these colonies can produce much higher concentrations of endotoxins.
The glycocalyx or capsule also serves as a similar structure to the fimbriae when defining important structures affecting the virulence of a certain microbe. The capsule is composed of polysaccharides and polypeptides, and coats the entire cell when present. The capsule itself is and organized layer of the two molecules, but the slime layer, which surrounds the capsule, is an unorganized mess of the two molecules. It is very loosely attached to the capsule layer of the cell. The capsule has many different features that affect the virulence of a cell. Firstly, the capsule acts as a defense against phagocytosis, as the engulfment of a heavily capsulated bacterial cell is ineffective and in some cases nearly impossible. The microbe's slippery glycocalyx slides off of the membranous surface during engulfment, whereas an unencapsulated bacterium would be susceptible to engulfment resulting in death. Secondly, the capsule prevents certain enzymes that would normally destroy the cell from entering the cell, therefore allowing the pathogen to survive and continue to cause disease.
Another feature of the capsule is that like fimbriae, it allows bacteria to attach to surfaces. The capsule makes attachment onto rocks in fast moving streams, plant roots, teeth, and human tissues possible for the microbe. This makes bacteria hard to physically remove from surfaces onto which they have attached, and allows more time for the release of toxins. The reason both the glycocalyx and the fimbriae are effective in the adherence to structures is the presence of ligands or adhesins. These ligands bind to complementary surface receptors, which form a bond. The adhesins that are generally present on a microbe are glycoproteins or lipoproteins. These adhesins generally bond with typical receptors on the host cell like sugars. Mannose is a common example of a receptor sugar.
Spores are developed in order to prevent bacterial death when adverse conditions occur. Endospores are created inside of a vegetative cell before they are released as Endospores. The spore is surrounded by a spore coat, which is a thick, hard protein layer. This provides the cell with considerable resistance when in contact with high temperatures and bactericidal chemicals. The spore also has a cell wall like the vegetative cell consisting of peptidoglycan. The cytoplasm of the spore has very high concentrations of dipicolinic acid, which is essential in the restart of metabolic reactions. The spore has less than 20% water in the cytoplasm, whereas a vegetative cell has around 80%. Tetanus bacteria demonstrate the survivability of spores, as they can remain dormant in the soil for long periods of time, and only restart metabolic processes once inside the human host, which provides excellent growth conditions.
Different species of bacteria have many structures that can help them to survive, and considerably varied methods of causing disease. The following examples of specific bacterial species demonstrate the varied nature of pathogenic bacteria. Tetanus is a disease caused by a bacterium called Clostridium tetani. The tetanus bacterium affects us by producing a toxin that attaches itself to the nervous system. The toxin makes its way to the spinal cord and binds to inhibitor neurons disrupting their function. Normally the inhibitor neurons keep the peripheral nerves from going out of control, but the toxin produced by tetanus allows those peripheral nerves to start firing faster and faster. All of these peripheral nerves firing faster and faster cause the muscles to tighten up. This stiffness in the muscles starts in the head and neck and spreads outwards until it reaches the limbs, eventually having a paralyzing effect. Lockjaw is a sign of a tetanus infection. It means that the Clostridium tetani toxins have started to stiffen the muscles in the jaw. Tetanus is a spore forming bacterium. This allows it to survive for long periods of time in conditions it would otherwise not survive. Tetanus is found, as a spore, in the soil and if any open cut that gets soil in it could potentially mean a tetanus infection. This structure, the spore, helps tetanus survive between hosts, where it would usually die. The toxins tetanus produces could also be considered a structure that helps tetanus to grow better. A human's immune system would normally fight off tetanus, if the body was functioning properly, but the toxin will eventually kill the human host, which will stop the body from fighting the infection. The tetanus bacteria could then grow uninhibited until all the body's nutrients were gone, then sporulate and wait for another host to come along. Clostridium tetani is a bacterium that affects the nervous system, but there are other body systems that can be affected by a given bacteria.
Escherichia coli is an example of a bacterium that affects the digestive system. E. coli also produces toxins, but they affect the host in a much different way. The toxins produced by E. coli can burst blood vessels lining the stomach and damage blood vessels throughout the body. The toxins can even cause potentially lethal kidney problems by causing blood clots to form in the blood stream that can clog up the kidneys and cause a condition known as hemolytic uremic syndrome. This clotting can also damage the heart, lungs, and the central nervous system. The toxin aids the growth E. coli in a unique way. If an antibiotic was used to kill the E. coli then the toxin it produces would spill out of the ruptured cells and into the stomach or intestines, causing a great deal of harm. This acts much in the same way as nuclear weapons, as a deterrent. In this way, antibiotics that rupture the cell wall would prove harmful instead of beneficial, thus indirectly aiding in the growth of E. coli.
Pneumonia is a disease that is mostly caused by one of two types of bacteria, Mycoplasm pneumoniae, or Streptococcus pneumoniae. Mycoplasmal pneumonia affects the mucous membranes of the lungs. It can produce an oxidizing agent that causes cell damage and it can also cause inflammation of the bronchi or alveoli. Streptococcal pneumonia affect the body in much the same way except that it is usually more severe, but only occurs in people whose immune system has been weakened. Pneumonia bacteria produce a capsule, which protects them from white blood cells. Bacteria are almost everywhere and with all of their various ways of surviving, infection seems inevitable.
It is impossible for an animal to avoid contact with microbes, many billions of which live harmlessly on the skin and in the gut, not to mention those that are breathed in or that penetrate the skin whenever it is pricked or cut. What is important is that microbes cannot be able to grow once inside the animal's body. Humans and other vertebrates have two sorts of defenses against microbial infection. One consists of the innate, or naturally present immune mechanisms, those that kill or inhibit a wide variety of microbes regardless of whether or not the body has had to fight them off before. This kind of immunity is non-specific; its mechanisms can act against microbes that are not necessarily similar to one another. The other type of defense mechanisms, in comparison, provides specific, acquired immunity. This form of immunity is specific in that its responses are tailored to act against a particular microbe or its products; it is acquired in that these tailor-made responses are enormously increased as a result of being stimulated by the previous presence of a given microbe or its products. Vaccination, or active immunization, gives protection by stimulating specific, acquired immunity.
Looking at the non-specific defenses of the host, the skin provides the first line of defense against invasion by microbes. Skin creates a mechanical barrier that is constantly replenished with new cells. Glands in skin cells secrete many substances that are harmful to bacteria, including lysozyme, which breaks down the cell walls of some bacteria, and oleic acid, a fatty acid that can prove fatal to certain bacterial species.
Within the body, the linings of the respiratory and gastrointestinal tracts both secrete mucus, which traps small particles, including bacteria. Protective antibodies (to be discussed later) are also present in the mucus. The linings of the stomach also secrete hydrochloric acid, at concentrations high enough to kill some bacteria. The stomach's gastric juices have a naturally very acidic pH (1.2 - 3.0), which also makes it unsuitable for the growth of microorganisms.
In the blood, white blood cells, or leukocytes, are the main defenses for the host. Those that employ phagocytosis as their means of destroying bacteria are known as phagocytes. These are usually neutrophils and macrophages, and are the only white blood cells that pertain to non-specific defenses. Attracted to pathogens by chemotaxis, a phagocyte will first adhere to a bacterial cell, and then its pseudopods will engulf the microorganism. The pathogen is then destroyed and digested with various enzymes. Some bacteria, by having a capsule, can make phagocytosis much more difficult, and cause digestive enzymes to be ineffective.
In some cases, fever result in response to a bacterial infection. Some bacterial species are so sensitive to high temperatures that the abnormally high bodily temperature that is produced can actually kill some bacteria, or at least put them in a state of bacteriostasis. Fever is often induced by bacterial endotoxins, usually produced by Gram negative bacteria.
A final non-specific defense of the host is what is known as the complement system. Antibodies, which are naturally present in many parts of the body will bind to foreign bacterial cells, and, after a number of enzymatic reactions, a transmembrane channel is formed through the cell wall of the microbe. This lesion allows for the leakage of the cellular contents (cytolysis), which will result in the death of the microorganism.
Specific, acquired immunity deals primarily with the binding of antigens and antibodies, and how lymphocytes, another kind of white blood cell, deal with pathogenic bacteria. There are two types of specific immunity: antibody-mediated, and cell-mediated. The latter is in response to intracellular antigens, which pertain to cell-invasive organisms like viruses, not to bacteria, and, for the purposes of this essay will be ignored. In the first type of specific defenses, humoral, or antibody-mediated immunity is regulated by B lymphocytes (or B cells). The inciting incident of the B cells' response is their initial encounter with new antigens. Antigens are simply chemical substances that cause the body to respond against the bacterium that produced them. They are found on the outer surface of bacterial cells, as well as the toxins they produce, and every strain of each bacterial species has its own specific antigen. This is why defenses that respond to antigens are specific, as they only pertain to one bacterial strain.
When an antigen is encountered, the corresponding B cell responds by cloning itself several times, and differentiating into memory cells and plasma cells. The spawned plasma cells secrete antibodies into the circulatory system, which will eventually bind with antigens, and suppress the virulence of the pathogen in a few ways. Firstly, the antibodies can bond to any toxins the bacterium may have produced, and effectively neutralize them. Secondly, antibodies can bond to the surface of the bacterial cells, tagging them for destruction by phagocytes. Agglutination will also occur, and the clumped bacteria that result will be much easier to engulf and destroy by phagocytes. Finally, when antibodies form a complement on the surface of the cell, this leads to the complement system, mentioned earlier, but which results in lysis of the bacterial cell.
The clones of the original B cells that were differentiated into memory cells can survive for years, and retain the specificity for the corresponding antigen. These memory cells are capable of quickly producing a new generation of plasma cells and the immune system can consequently field a large number of antibodies before the bacterial infection has had time to become widespread. As long as the memory cells survive, the organism has specific, acquired immunity to that particular pathogen.
The human immune system must be capable of suppressing the growth of foreign microorganisms in order to prevent the human body from becoming infected, and consequently has various methods for the destruction of harmful pathogens. At the same time, pathogenic bacteria have many structural characteristics that allow them to survive and grow in a human host. The structure of bacteria undeniably shows their tenacity and ability to grow, just as the human immune system is well made for its purpose of preventing infection, or destroying it when it occurs. It is for this reason that one can compare a pathogen and the immune system to two rivaling enemies, one always trying to best the other. In this case, however, their battle has been going on for millions of years, and will continue forever, until one becomes victorious.
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